1. Field of the Invention
The present invention generally relates to a system for controlling the speed of an electric motor.
2. Description of Related Art
Electric motors have been widely used in automotive applications. One common application using electric motors is an automotive fuel pump system. It is desirable to vary the amount of fuel provided from the fuel pump depending on the engine performance requirements. The amount of fuel delivered is dependent on the fuel pressure generated by the fuel pump. Generally, the fuel pressure is related to the speed of motor of the fuel pump. Typically most fuel pumps run at the highest pressure and maximum flow rate at all times to reduce the amount of fuel vapor for vehicle hot restart and provide sufficient fuel in a wide open throttle condition. However, running at the highest fuel pressure and flow is not efficient and negatively affects the life of the fuel pump.
One method used to vary motor speed to control fuel pressure uses a voltage drop resistor. The resistor is selectively connected to the motor voltage supply to control the voltage provided to the motor thereby changing the motor speed. Although this method reduces fuel pump wear, little energy is saved as the additional voltage is dissipated across the voltage drop resistor. Further, the additional heat energy created by the voltage drop resistor must be dissipated.
Another method used to vary motor speed thereby affecting fuel pressure includes modulating the driving signal. A pulse width modulator can be used to vary the duty cycle of the motor driving voltage thereby changing the motor speed. Although this method also reduces fuel pump wear and some energy is saved, the power and frequency of pulses required to drive the motor cause radio frequency interference problems for other vehicle components. Further, the use of a pulse width modulator in the control circuit increases system complexity and cost.
In view of the above, it is apparent that there exists a need for an improved system and method for controlling the speed of a motor.